Flashlamps are extensively used as a source of excitation radiation for energizing a laser medium to a lasing condition. For this purpose, the radiation in the flashlamp discharge arc or high energy plasma is typically focused by lenses or mirrors into the laser medium. It is thus necessary that the path of the arc discharge in the flashlamp between the energized electrodes be consistent or stable so that the point of focus of the discharge radiation will remain constant at the point of desired energization of the lasing medium.
Where it is desired to energize the laser medium for a series of high energy pulses a laser output, the flashlamp used to energize the medium will typically be a pulsed discharge as well. The path of the arc discharge between the electrodes of the flashlamp for each pulse of energization will, however, not normally be a constant where there is a large unbounded volume of gas within the flashlamp. Accordingly, such a lamp is unsatisfactory for use in exciting a laser medium for most applications.
The solution conventionally applied to this problem is to confine the discharge gas between the electrodes to a long narrow tube of quartz or glass such that the plasma created by the discharge is itself physically contained laterally between the electrodes giving rise to a more consistent arc discharge path. Such narrow tubes, however, introduce other difficulties for high pulse rate, short flash duration applications which shorten the lamp lifetime or reduces its efficiency. With such tubes, collection of deposits on the tube wall from erosion and sputtering of the electrodes and ultimately devitrification of the glass is more rapid. These in turn lead to greater heat absorption by the tube from discharge radiation and increase the already great thermal stress. The probability of tube explosion from shock wave effects in high rise time discharge is also aggravated.
A technique for stabilizing the discharge of a flashlamp and avoiding these other problems is to establish a vorticular flow of gas within a wider tube for the flashlamp by the introduction of a cool gas with a circumferential flow component. This effect works well for short arcs in the range of a few centimeters or for continuous arc discharges where the arc can be mechanically drawn from a short distance up to a longer distance of, for example, 20 centimeters.
Such a technique, however, is ineffective to stabilize long arcs of short duration pulsed discharges as is typically employed for pulsed laser energization. In such case, the long arc is desired in order to increase the impedance of the discharge and augment the energy radiated from it, while pulsed application is typically desired for lasers using a flowing medium such as a dye solution, the laser beam from which is useful in isotope separation. Such long, pulsed arcs will typically be unstable even with vortex stabilization in wider flashlamp tubes. This is due to the lack of a preexisting preferential path for each discharge pulse between the electrode such as caused by a temperature or pressure gradient resulting in a hot central region which would tend to confine the arc to the central line between the electrodes. Such a condition cannot be maintained between each pulse in high power discharges unless there is created a preexisting hot, plasma between the electrodes along the desired path for the discharge as suggested in an article by K. H. Hocker, entitled "Transient Discharges Across Vortex Stabilized Arcs" in a paper by the Institute fur Hochtemperaturforschung of the Technische Hochschule Stuggart, Germany. There, preionization for a discharge path is described in an experimental investigation of the heating of dense plasmas generated by the discharge of a capacitor bank across the preionized channel.